Conventionally, a threaded joint is widely used as a joint for OCTG. FIG. 1 is an axial sectional view schematically showing a general configuration of a threaded joint. As shown in FIG. 1, a threaded joint 100 comprises a pin 1, and a box 2 which is fastened to the pin 1. The pin 1 has an external thread part 11, a metal seal part 12 and a shoulder part 13 on the external surface. The box 2 has an internal thread part 21, a metal seal part 22 and a shoulder part 23 that correspond to the respective parts of the pin 1 on the internal surface.
The external thread part 11 and the internal thread part 21 (hereinbelow, these are generically referred to as the “thread parts 11, 21” as appropriate) perform a function for fastening the pin 1 and the box 2 by engaging with each other. The outside diameter of the metal seal part 12 is adapted to be slightly larger than the inside diameter of the metal seal part 22 (such difference between diameters is referred to as an “interference margin”). When the pin 1 is fastened to the box 2, the interference margin causes an interfacial pressure to occur in the contact portion of both metal seal parts 12, 22, and this interfacial pressure (contact interfacial pressure) satisfactorily holds the airtightness of the threaded joint 100. The shoulder parts 13, 23 perform a function for preventing the metal seal parts 12, 22 from generating so high a contact interfacial pressure as to cause an excessive plastic deformation, and securing a sufficiently large amount of screwing-in to make the fastening of the threaded joint 100 reliable. Some threaded joints not only provide an interference margin for the metal seal parts 12, 22, but also provide an interference margin similar to that for the metal seal parts 12, 22 for the thread parts 11, 21 in order to make the engagement between the thread parts 11, 21 reliable such that they will not be easily loosened. In this case, the shoulder parts 13, 23 also perform a function for restricting the interference between the thread parts 11, 21 to within the safe region, thereby suppressing generation of excessive stresses in the box 2.
As a method for evaluating the fastening state of a threaded joint having the above configuration, a method which monitors a change in torque generated at the time of fastening a threaded joint has conventionally found widespread use (for example, referring to JP 10-267175A). FIG. 2 is an explanatory drawing illustrating the conventional method for evaluating the fastening state of a threaded joint. As shown in FIG. 2, as fastening of a threaded joint is progressed in sequence, a torque is generated, resulting from a friction resistance due to an interference between the thread parts 11, 21, and an interference between the metal seal parts 12, 22. Then with the shoulder parts 13, 23 being butted against each other, the torque is abruptly raised. Conventionally, by the operator observing such a change in torque, whether or not the fastening state of the threaded joint is satisfactory has been determined. Specifically, for example, when the torque is raised to above a predetermined threshold value, the operator determines that the shoulder parts 13, 23 have butted against each other, and considers that the fastening of the threaded joint 100 has been satisfactorily completed.
However, the conventional evaluation method shown in FIG. 2 will not measure some physical amounts individually for determining whether the thread parts 11, 21 actually interfere with each other, whether the metal seal parts 12, 22 actually interfere with each other, and whether the shoulder parts 13, 23 are actually butted against each other, respectively. It is an evaluation method which is based on an empirical criterion that the torque generation may be achieved by the tight contact of parts (by interference or butting). Certainly, a torque is generated through the tight contact of parts (by interference or butting). However, in such a case as that where the thread parts 11, 21 have had a seizure, some other factor can generate a great torque, thus simply by monitoring a change in torque, it is difficult to accurately evaluate the fastening state.
In addition, the conventional evaluation method shown in FIG. 2 is subjected to a constraint that it is required to continuously monitor the torque in the course of fastening the threaded joint (while the pin and the box being subjected to a relative movement to be fastened) (because, in a state subsequent to the fastening where the pin and the box have come to a standstill, the fastening state cannot be evaluated).
On the other hand, in some cases, whether the shoulder parts 13, 23 are butted against each other is determined by whether or not a clearance gage having a thickness of 0.1 mm can be inserted between the shoulder parts 13, 23. If the clearance gage 0.1 mm thick can be inserted between the shoulder parts 13, 23, it is determined that the shoulder parts 13, 23 are not butted against each other, which indicates the fastening state is unsatisfactory.
However, the clearance which allows insertion of a clearance gage 0.1 mm thick is as large as 0.15 mm or more, and thus there arises a problem that whether or not a clearance smaller than that is given cannot be determined.
In addition, in order to insert a clearance gage 0.1 mm thick into a clearance of 0.15 mm, it is required to position the clearance gage on a plane along this clearance and carefully insert it. Specifically, it is required to insert the clearance gage into the threaded joint from one opening end of the threaded joint (in other words, to move the clearance gage in the axial direction of the threaded joint) for positioning the clearance gage on a plane along this clearance which is followed by moving the clearance gage toward the clearance on the plane. Thus, there occurs a problem that evaluation using a clearance gage requires a lot of manpower, being time consuming.